Polyethylene terephthalate based copolyesters (PET) have been widely used to make containers for carbonated soft drink, juice, water and the like due to their excellent combination of clarity, mechanical and gas barrier properties. However, the use of PET containers for the carbonated soft drink (CSD) has been limited due to the fact that CO2 can permeate through a PET container fairly quickly. The permeation rate of the CO2 in a CSD through a PET container at room temperature is in the range of 3 to 20 cc/day depending on the size of the container, or at a relative loss rate of 1.4 to 2.5%/week when normalized to the starting CO2 level. The relative loss rate depends on the container size, or rather the surface area to volume ratio. The higher the surface area to volume ratio, the higher the relative loss rate. A smaller sized container has a larger surface area/volume ratio thus resulting in a higher relative loss rate. For this reason, PET containers are currently used only as larger sized containers for CSD while metal cans and glass containers are the choice of the smaller sized packages.
The shelf life of a bottled CSD is determined by the amount of CO2 remaining in the beverage. Normally for a CSD, the containers are filled with a CO2 level of 4 volumes CO2/volume H2O (which is conveniently called 4 volumes of CO2). When 17.5% of CO2 in the bottle is lost or a CO2 level of 3.3 volumes is reached due to CO2 permeation through the container sidewall and closure, the product reaches the end of its shelf life. In the case of beverages with lower carbonation levels, a CO2 level of 2 to 2.5 volumes CO2/volume H2O is normally required and a certain amount of CO2 loss marks the shelf life of the products. In all the cases, the amount of CO2 left in the container determines the shelf life of the beverage and thus the suitability of PET as a packaging material.
To prevent the CO2 loss, there have been many barrier technologies developed or being developed that try to enhance the barrier of the PET containers to small molecules such as CO2. Regardless of the mechanisms, these barrier technologies all intend to slow down the permeation of CO2 through the container sidewall or slow down the loss of CO2 inside the container. This, however, does not change the total amount of CO2 that the beverage can afford to lose for the beverage product to have an acceptable quality. For example, for a 500 ml bottle filled with 4 volumes of CO2/volume of water, the amount of CO2 loss that can be tolerated before the product reaches its maximum shelf life is 350 ml. The barrier technologies only extend the time it takes this amount of CO2 loss through the sidewall. The total amount of tolerable CO2 loss, 350 ml, will not change based on different barrier technologies used. In addition, almost all of the practically available barrier technologies today require capital investment and add substantial cost to container manufacturer.
GB 0 970 376 discloses a beverage with an added bactericide that hydrolyzes to form CO2 and a compatible alcohol. A suitable bactericide is pyrocarbonic acid diethylester which hydrolyzes in an aqueous solution to form ethyl alcohol and carbonic acid. The disclosure claimed that a beverage treated with this method had an extended shelf life. The rate of the carbonation release for these additives in a carbonated beverage, however, is substantially higher than the loss of the CO2 through permeation. The sudden rise of the CO2 level is therefore beyond the acceptable quality level.
U.S. Pat. No. 5,855,942 discloses a method and composition for enhancing the retention of CO2 in carbonated beverages via addition of a carbonic acid ester in the beverage. The carbonic acid esters release CO2 through the acid catalyzed hydrolysis of the carbonic acid ester in the acid aqueous environment of the carbonated beverage. The release of CO2 is claimed to occur at the similar rate of CO2 permeation through the sidewall.
The above technologies, while generating CO2 to compensate the CO2 loss through the sidewall and closure, are not practical and are very difficult to use. The addition of any compound in the beverage alters the beverage composition. Alteration of the beverage not only dramatically affects the taste and the nature of the beverage, but the added compounds also have to be compatible with the beverage product so that no solid deposits form and cloud the beverage product. Changing the beverage composition can also create regulatory issues, if the additives are not compliant with the regulations or form toxic by-products as a result of the reaction.
Thus, there remains a need for a simple and effective system of compensating for CO2 loss in packaged CSD without adversely affecting the CSD composition.